<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Vue 测试实例 - 菜鸟教程(runoob.com)</title>
    <script src="https://cdn.staticfile.org/vue/2.4.2/vue.min.js"></script>
</head>
<body>
<div id="computed_props">
    千米 : <input type="text" v-model="kilometers">
    米 : <input type="text" v-model="meters">
</div>
<p id="info"></p>
<script type="text/javascript">
    var vm = new Vue({
        el: '#computed_props',
        data: {
            kilometers: 0,
            meters: 0
        },
        methods: {},
        computed: {},
        watch: {
            kilometers: function (val) {
                this.kilometers = val;
                this.meters = this.kilometers * 1000
            },
            meters: function (val) {
                this.kilometers = val / 1000;
                this.meters = val;
            }
        }
    });
    // $watch 是一个实例方法
    vm.$watch('kilometers', function (newValue, oldValue) {
        // 这个回调将在 vm.kilometers 改变后调用
        document.getElementById("info").innerHTML = "修改前值为: " + oldValue + "，修改后值为: " + newValue;
    })
</script>
</body>
</html>